Luminescent compounds are well known security elements for the protection of banknotes, valued papers and other security articles. Such compounds may be incorporated into the security article's substrate, printed onto security articles via an ink, or affixed to security articles in the form of a security thread, a foil or a label carrying them.
The detection of luminescent security elements is well known in the art and described in a large number of patents. U.S. Pat. No. 5,918,960 describes a counterfeit banknote detecting apparatus, based on a UV lamp to excite the luminescence, and two photocells, to measure the luminescence intensity versus the background radiation intensity. A particular problem in luminescence detection is the discrimination of the weak luminescence signal from the often much stronger background signals, which are due to environmental light. The use of modulated excitation and synchronous detection has been proposed as a possibility to overcome this difficulty.
U.S. Pat. No. 5,608,225 describes an improved fluorescent detecting apparatus and a method using a modulated excitation source, a photocell, and a phase detector, for the suppression of background signals. U.S. Pat. Nos. 4,275,299, 5,548,106, 5,418,855 and 5,574,790 describe further detection equipment based on modulated excitation. U.S. Pat. No. 3,656,835 teaches the joint use of a constant UV-excitation source and a modulated magnetic field, to produce and detect modulated emission from magnetic triplet states of the luminescent. U.S. Pat. Nos. 5,315,993 and 5,331,140 propose luminescence decay monitoring using a multiplexing of more than one modulation frequency of the excitation source, e.g. for the reading of invisible fluorescent barcodes. U.S. Pat. Nos. 5,548,124 and 5,757,013 propose the measurement of luminescence decay times through the generation of a modulation product of the excitation signal and the back-received luminescent response signal.
The modulation-based luminescence detection systems of the prior art are quite rugged against environment light influences which do not have the same modulation frequency and phase as the detector's own light source. They are, on the other hand, very sensitive to their own modulation frequency. Some of the modulated excitation light is noteworthy back-scattered at the sample surface and leaks through the optical filter system into the detector's photocell. No optical filter system has noteworthy a 100% rejection of the off-band light components. This residual excitation light, which has exactly the same frequency as the luminescence response, adds thus to the detected signal intensity. In the case of a weak luminescence signal, said background signal impedes a proper determination of the luminescence signal intensity.
This is the more disturbing as the background signal depends on the reflectivity of the substrate, which may vary independently of the luminescence signal intensity. In the case of banknote authentication, the substrate reflectivity depends noteworthy on external factors such as dirtiness and wear, which makes it difficult to check the banknote fore genuineness if no distinction can be made between the merely reflected background signal and the true luminescence emission signal.
The present invention discloses a method and an equipment which overcome the shortcomings of the prior art.
In particular it discloses a method and an equipment which allow to discriminate between the reflected excitation signal and the luminescence emission signal, and to determine selectively the strength of the luminescence emission.
The present invention allows furthermore for a quantitative determination of luminescence intensity, independent of background reflectivity.
It allows further to derive absolute or comparative luminescence intensities, and to exploit these for coding and identification purposes.